Day 1791

Monday, Tuesday, Wednesday.

2101.03699

Precise photometric measurements from a 1903 photographic plate using a commercial scanner

Cenry, et al

We demonstrate the feasibility of determining magnitudes of stars on archival photographic plates using a commercially available scanner. We describe one photometric approach that could serve as a useful example for other studies. In particular, we measure and calibrate stellar magnitudes from a 1903 photographic plate from the Yerkes Observatory collection, and demonstrate that the overall precision from our methods is better than 0.10 mag. Notably, these measurements are dominated by intrinsic plate noise, rather than noise introduced through the scanning/digitization process. The low expense of this approach expands the scientific potential to study variable stars in the archives of observatory plate collections. We use the serendipitous discovery of a candidate transient at photographic magnitude pg = 16.60 in the spiral galaxy NGC 7331 to illustrate our photometric methods. If this unknown source is a supernova, it would represent the fourth known supernova in NGC 7331.

2101.03723

The PAU survey: narrowband photometric redshifts using Gaussian processes

Soo, et al

We study the performance of the hybrid template-machine-learning photometric redshift (photo-z) algorithm Delight on a subset of the early data release of the Physics of the Accelerating Universe Survey (PAUS). We calibrate the fluxes of the 40 PAUS narrowbands with 6 broadband fluxes (uBVriz) in the COSMOS field using three different methods, including a new method which utilises the correlation between the apparent size and overall flux of the galaxy. We use a rich set of empirically derived galaxy spectral templates as guides to train the Gaussian process, and we show that our results are competitive with other standard photometric redshift algorithms, including the bespoke template-based BCNz2 previously applied to PAUS, lowering the photo-z 68th percentile error to σ68≈0.009(1+z) without any quality cut for galaxies with iauto<22.5. We use the combined Delight and BCNz2 results to identify a small number of tentative catastrophic failures among the secure spectroscopic redshift measurements in zCOSMOS. In the process, we introduce performance metrics derived from the results of both algorithms which improves the photo-z quality such that it achieves σ68<0.0035(1+z) at a magnitude of iauto<22.5 while keeping 50 per cent objects of the galaxy sample.

2101.04675

How not to obtain the redshift distribution from probabilistic redshift estimates: under what conditions is it no inappropriate to estimate the redshift distribution N(z) by stacking photo-z PDFs?

Malz

The scientific impact of current and upcoming photometric galaxy surveys is contingent on our ability to obtain redshift estimates for large numbers of faint galaxies. In the absence of spectroscopically confirmed redshifts, broad-band photometric redshift point estimates (photo-zs) have been superseded by photo-z probability density functions (PDFs) that encapsulate their nontrivial uncertainties. Initial applications of photo-z PDFs in weak gravitational lensing studies of cosmology have obtained the redshift distribution function N(z) by employing computationally straightforward stacking methodologies that violate the laws of probability. In response, mathematically self-consistent models of varying complexity have been proposed in an effort to answer the question, "What is the right way to obtain the redshift distribution function N(z) from a catalog of photo-z PDFs?" This letter aims to motivate adoption of such principled methods by addressing the contrapositive of the more common presentation of such models, answering the question, "Under what conditions do traditional stacking methods successfully recover the true redshift distribution function N(z)?" By placing stacking in a rigorous mathematical environment, we identify two such conditions: those of perfectly informative data and perfectly informative prior information. Stacking has maintained its foothold in the astronomical community for so long because the conditions in question were only weakly violated in the past. These conditions, however, will be strongly violated by future galaxy surveys. We therefore conclude that stacking must be abandoned in favor of mathematically supported methods in order to advance observational cosmology.

Day 1790

Wednesday, Thursday, Friday.

2101.01281

Laboratory measurements of instrumental signatures of the LSST camera focal plane

Snyder, et al

Electro-optical testing and characterization of the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) Camera focal plane, consisting of 205 charge-coupled devices (CCDs) arranged into 21 stand-alone Raft Tower Modules (RTMs) and 4 Corner Raft Tower Modules (CRTMs), is currently being performed at the SLAC National Accelerator Laboratory. Testing of the camera sensors is performed using a set of custom-built optical projectors, designed to illuminate the full focal plane or specific regions of the focal plane with a series of light illumination patterns: the crosstalk projector, the flat illuminator projector, and the spot grid projector. In addition to measurements of crosstalk, linearity and full well, the ability to project realistically-sized sources, using the spot grid projector, makes possible unique measurements of instrumental signatures such as deferred charge distortions, astrometric shifts due to sensor effects, and the brighter-fatter effect, prior to camera first light. Here we present the optical projector designs and usage, the electro-optical measurements and how these results have been used in testing and improving the LSST Camera instrumental signature removal algorithms.

2101.01542

Starlet L1-norm for weak lensing cosmology

Ajani, et al

We present a new summary statistic for weak lensing observables, higher than second order, suitable for extracting non-Gaussian cosmological information and inferring cosmological parameters. We name this statistic the 'starlet ℓ1-norm' as it is computed via the sum of the absolute values of the starlet (wavelet) decomposition coefficients of a weak lensing map. In comparison to the state-of-the-art higher-order statistics -- weak lensing peak counts and minimum counts, or the combination of the two -- the ℓ1-norm provides a fast multi-scale calculation of the full void and peak distribution, avoiding the problem of defining what a peak is and what a void is: The ℓ1-norm carries the information encoded in all pixels of the map, not just the ones in local maxima and minima. We show its potential by applying it to the weak lensing convergence maps provided by the MassiveNus simulations to get constraints on the sum of neutrino masses, the matter density parameter, and the amplitude of the primordial power spectrum. We find that, in an ideal setting without further systematics, the starlet ℓ1-norm remarkably outperforms commonly used summary statistics, such as the power spectrum or the combination of peak and void counts, in terms of constraining power, representing a promising new unified framework to simultaneously account for the information encoded in peak counts and voids. We find that the starlet ℓ1-norm outperforms the power spectrum by 72% on Mν, 60% on Ωm, and 75% on As for the Euclid-like setting considered; it also improves upon the state-of-the-art combination of peaks and voids for a single smoothing scale by 24%on Mν, 50% on Ωm, and 24% on As.

2101.01723

Completeness of the Gaia-verse IV: the astrometry spread function of Gaia DR2

Everall, et al

Gaia DR2 published positions, parallaxes and proper motions for an unprecedented 1,331,909,727 sources, revolutionising the field of Galactic dynamics. We complement this data with the Astrometry Spread Function (ASF), the expected uncertainty in the measured positions, proper motions and parallax for a non-accelerating point source. The ASF is a Gaussian function for which we construct the 5D astrometric covariance matrix as a function of position on the sky and apparent magnitude using the Gaia DR2 scanning law and demonstrate excellent agreement with the observed data. This can be used to answer the question `What astrometric covariance would Gaia have published if my star was a non-accelerating point source?'. 
The ASF will enable characterisation of binary systems, exoplanet orbits, astrometric microlensing events and extended sources which add an excess astrometric noise to the expected astrometry uncertainty. By using the ASF to estimate the unit weight error (UWE) of Gaia DR2 sources, we demonstrate that the ASF indeed provides a direct probe of the excess source noise. 
We use the ASF to estimate the contribution to the selection function of the Gaia astrometric sample from a cut on astrometric_sigma5d_max showing high completeness for G<20 dropping to <1% in underscanned regions of the sky for G=21. 
We have added an ASF module to the Python package SCANNINGLAW (https://github.com/gaiaverse/scanninglaw) through which users can access the ASF.

2101.01725

Predicting the observability of the population III stars with ELT-HARMONI via the Helium 1640A emission line

Grisdale, et al

Population III (Pop. III) stars, as of yet, have not been detected, however as we move into the era of extremely large telescopes this is likely to change. One likely tracer for Pop. III stars is the HeIIλ1640 emission line, which will be detectable by the HARMONI spectrograph on the European Extremely Large Telescope (ELT) over a broad range of redshifts (2≤z≤14). By post-processing galaxies from the cosmological, AMR-hydrodynamical simulation NewHorizon with theoretical spectral energy distributions (SED) for Pop. III stars and radiative transfer (i.e. the Yggdrasil Models and CLOUDY look-up tables respectively) we are able to compute the flux of HeIIλ1640 for individual galaxies. From mock 10 hour observations of these galaxies we show that HARMONI will be able to detect Pop. III stars in galaxies up to z∼10 provided Pop. III stars have a top heavy Initial Mass Function (IMF). Furthermore, we find that should Pop. III stars instead have an IMF similar to those of the Pop. I stars, the HeIIλ1640 line would only be observable for galaxies with Pop. III stellar masses in excess of 107M⊙yr−1, average stellar age <1Myr at z=4. Finally, we are able to determine the minimal intrinsic flux required for HARMONI to detect Pop. III stars in a galaxy up to z=10.

2101.02623

Automatic identification of outliers in Hubble Space Telescope galaxy images

Shamir

Rare extragalactic objects can carry substantial information about the past, present, and future universe. Given the size of astronomical databases in the information era it can be assumed that very many outlier galaxies are included in existing and future astronomical databases. However, manual search for these objects is impractical due to the required labor, and therefore the ability to detect such objects largely depends on computer algorithms. This paper describes an unsupervised machine learning algorithm for automatic detection of outlier galaxy images, and its application to several Hubble Space Telescope fields. The algorithm does not require training, and therefore is not dependent on the preparation of clean training sets. The application of the algorithm to a large collection of galaxies detected a variety of outlier galaxy images. The algorithm is not perfect in the sense that not all objects detected by the algorithm are indeed considered outliers, but it reduces the dataset by two orders of magnitude to allow practical manual identification. The catalogue contains 147 objects that would be very difficult to identify without using automation.